This invention relates to the field of protecting electrical circuits against potentially damaging effects of electrical transients and, more particularly, to methods and apparatus for protecting AC-powered user equipment from both overvoltage and undervoltage conditions.
Many types of electrical equipment are susceptible to malfunctions or damage due to transient impulses or voltage conditions which exceed an acceptable voltage range or window. For example, computers and TV satellite receiver decoders are very sensitive to voltage transients. AC powered motors are subject to damage from overheating when driven by AC voltages greater than or less than an intended AC voltage operating range.
Since voltage transients may be caused by such things as lightning strikes, inductive load switching and physical shock to power lines, utility companies which supply electrical power have no practical ways of preventing such occurrences. The problems are particularly acute in rural areas.
Various crowbar circuits are known for shunting an AC power line to ground in the event of an overvoltage condition. Crowbar circuits may employ, for example, gas discharge tubes or SCR's connected across the power line. A crowbar circuit, when fired, presents an essentially short circuit across the power line, thereby causing a fuse to blow or a circuit breaker to trip upstream of the crowbar circuit to disconnect the AC line from the user equipment. A user must locate, identify and replace or reset the fuse or circuit breaker, respectively, in order to restore AC power to the user equipment. These actions are inconvenient and extend the equipment downtime.
Various solid state transient protection components also are known, including zener diodes or silicon avalanche diodes, varistors including the metal oxide varistor, and the like. Typically, such components are connected across the AC power line to shunt transient signals that exceed a predetermined "clamping" voltage. They may be used in combination, as taught in U.S. Pat. Nos. 4,571,656 and 4,156,838. When shunt components are used across the AC power line, the user equipment remains connected to the AC power line at all times. The user equipment is left exposed to potential damage to the extent that a transient spike or surge is not absorbed by the shunt devices.
Many shunt-type protective circuits have clamping voltages that are not well defined or are too high. For example, where successive shunt stages are used, as shown in U.S. Pat. No. 4,571,656, the impedance to ground decreases gradually as a powerline spike increases in voltage and power, due to the respective turn-on characteristics of each of the shunting devices. The final shunt stage may not fully turn on until the powerline voltage exceeds 200 VAC or more. Most AC-powered user equipment in the U.S. is designed to operate at a nominal line voltage of 120 VAC +/- 10% (i.e. 108-132 volts). Shunt circuits therefore do not adequately protect such user equipment from overvoltage conditions.
Shunt devices also subject user equipment to erratic on-again, off-again operation where the AC power line voltage is irregular. This intermittant operation itself can lead to potentially damaging transients, particularly if the user equipment load is inductive. For example, intermittent operation of electric motor starter windings causes them to burn out.
Another drawback of shunt device circuit protection is that a powerful powerline transient may destroy a solid state shunt device, effectively removing it from the circuit. The user equipment is left completely unprotected against any subsequent transients.
Finally, shunt devices offer no protection against undervoltage conditions, i.e., where the AC line voltage falls below a predetermined minimum voltage, for example, 90 volts. This is frequently referred to as a "brown-out" condition. Motors connected to refrigerant compressors and similar loads can burn out under brownout conditions. Computer disk drive damage can also be caused by brownouts.
U.S. Pat. No. 4,689,713 discloses a high voltage surge protection circuit that includes a peak limiting bridge. In operation, an overvoltage transient is shunted to ground through a power diode and capacitor, connected in series. The peak limiting bridge suffers the same shortcomings as the other shunt devices described above.
Accordingly, a need remains for an improved AC voltage monitor and controller, particularly for general purpose protection of AC-powered equipment.